The use of electromagnetic measurements in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. Moreover, azimuthally sensitive directional resistivity measurements are commonly employed, e.g., in pay-zone steering applications, to provide information upon which steering decisions may be made. Directional resistivity tools often make use of tilted or transverse antennas (antennas that have a magnetic dipole that is tilted or transverse with respect to the tool axis).
Tool calibration is an important and necessary task in electromagnetic logging operations. Factors such as imperfections in tool construction and variations due to tool electronics can introduce significant measurement errors. The intent of tool calibration is to eliminate and/or compensate for the effects of these factors on the measurement data. Various tool compensation methods are known. For example, air calibration methods are commonly employed. In such methods, an electromagnetic resistivity tool may be suspended (e.g. via crane) in air away from any conducting media. A resistivity measurement should yield near-infinite resistivity (i.e., a conductivity of zero). Any deviation is subtracted and is assumed to be related to systematic measurement errors (e.g., related to the electronics, hardware, or processing methods). U.S. Pat. Nos. 4,800,496 and 7,027,923 disclose methods for determining a sonde error in induction or propagation logging tools that require measurements to be made at two or more heights above the surface of the earth.
While the aforementioned calibration methods may provide an adequate calibration for conventional electromagnetic logging tools, they can be difficult to implement with deep reading, directional electromagnetic resistivity tools (look-around tools) or electromagnetic look-ahead tools. As described in more detail below, the transmitter and receiver subs in such deep reading tools are modular such that neither the axial spacing nor the azimuthal alignment angle between the subs are fixed. Hence a calibration performed for one tool configuration will not necessarily be valid for any other tool configuration. Moreover, performing a conventional air calibration tends to be difficult if not impossible to implement at a drilling site owing to the long spacing between transmitter and receiver subs (e.g., up to 100 feet or more) and the need to suspend the entire BHA. Therefore, there remains a need in the art for an improved system for calibrating directional resistivity logging tools.